Bottom-hole pressure measuring and continuous monitoring in particular are invaluable in the management of oil and gas wells for fiscal projections, production exploitation, and the prevention of well or formation damage that can prematurely end the productive life of a hydrocarbon reservoir. Real-time pressure monitoring is essential to the prevention of costly service intervention in high capacity, deep-water, remote, and sub-sea wells. Elaborate and often expensive systems are deployed for the dedicated purpose of down-hole pressure monitoring. The typical preload of a conventional back-check valve or pair of valves designed for use in a down-hole chemical injection mandrel yields between 60 to 130 pounds per square inch. The hydrostatic weight of fluid combined with injection pressure typically present excessive forces that easily overcome the back-check valve spring load during even infinitesimal reductions in down-hole pressure.
Methods for monitoring down-hole pressures without interruption of production or injection operations were first tested in Germany several years ago. This initial development and its subsequent modifications required electric cable to transmit a signal reflecting down-hole pressures.
Bottom-hole pressure data are routine requirements for evaluation of production and reservoir performance. Monitoring of reservoir-pressure response may be especially helpful in evaluation and control of supplemental recovery projects. This might include producing, buildup, and static surveys as determined by pressure recorders run on wire line. However, frequency and number of wells conventionally surveyed may be limited due to interruption of normal production routine, as well as the expense of such interruptions. Presence of some artificial-lift equipment will prevent running conventional pressure surveys. Furthermore, production of highly corrosive fluids, together with potential damage from wire-line cutting where plastic-coated tubing is installed, can also be a deterrent to obtaining useful pressure data.
Where the expense can be justified, installation of permanent bottom-hole pressure monitors offers a means of securing such data.
Electrical methods, such as strain gauges to measure pressure, have been available in several forms for many years. In 1998, a taut wire gauge was developed and first received widespread use in Europe. The ends of the taut wire are attached to a sealed steel housing and a steel diaphragm. A current pulse transmitted down hole energizes the wire. As pressure is applied to the diaphragm, tension in the wire is changed with accompanying changes in natural frequency of the wire. An electrical signal is transmitted to a surface receiver for comparison with a signal from a standard calibrated wire for determination of the applied pressure. Detailed description of this equipment plus practical applications in the Rocky Mountain area has been well documented.
During 1959, a down-hole bourbon tube-type gauge was developed in the United States. As pressure is applied to the spiral formed tube, coupled to a code wheel made of an electrical conductor and an electrical insulator, a pattern change in current requirements is affected. By decoding the current pattern, the bottom-hole pressure can be determined.
In each of these methods, down-hole signals are transmitted to the surface by means of an electrical cable, which is normally attached to the exterior of the production or injection tubing.
More recently, a pressure gauge using a quartz transducer rather than a taut wire has become available for field applications. In even more recent developments, new tools have been introduced which do not use any down-hole electronics or electrical conduits by using a pressure-transmission system consisting of a 3/32-in. I.D. capillary tube attached to the outside of the production tubing. This small capillary tube connects a surface recorder to a down-hole chamber in communication with the well fluids.
In the pressure-transmission approach, a down-hole chamber is connected to a surface monitor by a small-diameter tube filled with a single-phase gas, usually nitrogen. The tube is normally secured to the outside of the production tubing, extended through a packing gland in the casing head, then to a surface-pressure recorder and optional digital readout unit. The down-hole chamber permits expansion and compression of the pressure-transmitting gas without entry of well fluids into the tube (FIG. 1). The size of the chamber is dependent on the anticipated pressure range to be encountered and the diameter of the tube. The capillary tube type is dependent on the down-hole environment. A protector or guard banded to the production tubing covers the tube at each collar.
As compressibility will vary with pressure and temperature, which also vary with depth, corrections must be provided for changes in these conditions throughout the anticipated pressure range to be recorded. A portable monitor and printer are generally used with the pressure-type monitoring system. As a side benefit, the combination monitor and printer can also be used for the recording of surface buildups or other pressure monitoring on wells which have no down-hole detector.
Wire-line pressure surveys are often run in permanent pressure monitored wells to determine the reliability of the results obtained with the permanent pressure transmission systems. Calibration is then required by adjusting the gas pressure in the capillary tube to compensate for the errors. Since pressure is sensitive to the prevailing temperature, it is essential that accurate temperature monitoring be achieved. Therefore, in current permanent pressure monitoring systems of this type, errors are prominent, especially in deep wells, and must be compensated for in the recording system by extrapolation.
In addition, tubing hanger penetration limitations often don't allow for the development of an electronic or optical down-hole pressure gauge.
The initial expense of permanent down-hole pressure monitors is greater than routine wire-line pressure surveys with installation expense varying with depth.
As a result of the expense and inefficiencies of the above-related systems, a more effective and less expensive permanent down-hole pressure monitoring system has been developed and disclosed herein.